The truck that ran into a Bastille Day celebration Thursday night in Nice, France, killing dozens, appeared to have done so in a well-planned, intentional attack, witnesses and security experts told NBC News.

Nader El Shafei, 42, was vacationing from Cairo, in the lobby of the Hotel Mercure on the Promenade des Anglais, watching holiday fireworks when he saw the truck.

"He smashed into a lot of people," El Shafei told NBC News. "I was waving to the driver, 'Stop' there's a girl under the truck.'"

He said he still thought it might have been an accident at that point, but then realized it was likely a deliberate attack. "He was not concentrating on what was going on. He was looking back and forth," said El Shafei.

"I saw him pick up a phone, I thought, and at this point I still think it's an accident and then I see he pulls out a gun. It looked like a handgun, a Glock. He pulled it out and I understood something was wrong...and then I see the police shooting him."

As everyone started to flee, he remained still.

"I couldn't run," he said. "I was just frozen."

NBC News terrorism security analyst Malcolm Nance, executive director of the Terror Asymmetrics Project, a nonprofit research institute, said the incident bore all the hallmarks of a "major terror attack."

Vehicular traffic is barred from the promenade on holidays, meaning the driver of the truck would have had to negotiate several roadblocks to reach the area, said Nance, a former veteran federal counterterrorism officer for the Department of Homeland Security.

Nance said the driver used a strategy often used by al Qaeda in the Arabian Peninsula and by attackers in Israel — zigzagging through the crowd and then speeding up when authorities tried to pull him over.

The incident started as thousands of people were leaving the beach about 20 minutes after a large fireworks display to celebrate France's national holiday, Motamedi said.

"We were actually talking about how that basically would have been the perfect setup for a terrorist attack, because you had wild explosions, and it's hard to differentiate gunfire from fireworks from a bomb going off," he said.

People cross the street with their hands on their heads early Friday as a soldier secures the scene where a truck drove into throngs of pedestrians along the Promenade des Anglais in Nice, France.Jean-Pierre Amet / Reuters

"For something like this to have happened, especially right after the fireworks, clearly it was a very well-coordinated, well-timed event, because it certainly was no accident," Motamedi said, adding: "It was the scariest moment of my life, easily."

A woman identifying herself as Coralie told France 24 News that a "very, very big truck" began running people over.

"I pushed my mother into a building along with about 50 other people. When I reached the door, shots were fired," she told the station.

"We left the kids' strollers on the ground floor, carried the kids in our arms and we waited," she said. "We thought it's not possible, but people were yelling 'It's a terrorist attack!' when the truck started running people over.

"Then more shots were fired."

#nice saw moving truck driving full speed over crowd on promenade for at least 2-3 miles. Then massive gunfire.

Sophie Sanderlands, an Australian woman who was on vacation in Nice, told Australia's Seven Network that the scene went from a "really happy mood" to absolute chaos.

"We were down on the beach front enjoyoing the fireworks that they put on for the celebration nothing seemed out of place five or six minutes after it finished, started walking down," she told the network. " People everywhere, really happy mood, nothing was out of place, and then all of a sudden, thousands of people just started running towards us, screaming, crying, absolutely out of control."

She said "there were strollers on the ground, people crammed into restaurants, people trying to into restaurants but they were shutting them out because there was no room left, pulling curtains across windows."

Linda Hervieux reported from Nice, France

Alex Johnson

Alex Johnson is a senior writer for NBC News covering general news, with an emphasis on explanatory journalism, data analysis, technology and religion. He is based in Los Angeles.

Erin Calabrese

Emmanuelle Saliba, Euronews

Emmanuelle Saliba is head of The Cube, a studio and news desk designed to discover, debunk and verify information in real-time at Euronews.